The present invention relates to trimmable capacitors formed in multilayer metallisation/dielectric structures such as those used in multichip modules (MCM).
The multichip module approach to interconnection and packaging employs unpackaged integrated circuit (IC) dice mounted onto a high interconnection-density substrate structure, the resulting assembly being packaged to form a module. The removal of the individual die packaging overhead allows very close die proximity (from 0.4 to 2 mm die spacings being typical) and a high functional density to be achieved. This technology also provides a considerable area and weight saving over the equivalent circuit function realised using individually packaged ICs and surface mount assembly. Typical area savings of between 5 and 10 fold at the substrate level may be achieved. The area fraction of functional silicon in this technology when measured at the substrate level is a function of the assembly method employed. For wire bonded modules this area fraction is typically 30 to 60 percent, while flip chip solder bonded module assemblies can provide area fractions of up to 80 percent. Improvements in reliability are also achieved through the elimination of package leads, solder joints and other materials interfaces in the transition from individual to multichip packaging. Performance improvements are achieved through the elimination of individual device package parasitics and the typically three-fold reduction in inter-device trace lengths.
A number of categories of MCM technology have been defined, describing the type of structure employed to interconnect the naked dice in the MCM assembly. The MCM-L category uses very fine line, multilayer printed circuit board (pcb) substrate structures that are fabricated using extensions of existing pcb manufacturing techniques. Such substrates are suited for interconnect only, all digital modules with low or medium power dissipation and medium wiring density demand (i.e. medium device pin count). They offer the lowest cost per unit area of the MCM substrate alternatives and employ typically up to six metal and dielectric layers. The MCM-C category uses cofired, multilayer ceramic technologies to provide the required interconnection structure. Since the track geometries in this technology are commonly defined by screen printing, which implies limited resolution, the required routing density is achieved by employing additional layers in the multilayer substrate structure. MCM-C modules with up to 40 layers have been reported. The MCM-C substrate structure may be readily incorporated with the module package structure to provide a reliable, cost effective approach to MCM construction for many all-digital, high reliability applications.
The MCM-D substrate category that is of interest in the present application is constructed by thin film deposition technologies (as indicated by the -D qualification) for example with a multilayer aluminium-polyimide metallisation structure (as typical of a silicon MCM substrate), with typically three or four metallisation layers, a ground plane, a power plane and one or two layers for signal trace routing. A five layer construction is also sometimes employed, the fifth metallisation layer being used to place chip connection pads, thus avoiding any constraint on routing in the two signal routing layers below.
The tracks on the silicon substrate are of widths between 10 and 25 micrometers, with metal thicknesses of 2 to 5 micrometers at track pitches of 40 to 100 micrometers, dielectric thicknesses being in the 5 to 20 micrometer range. Such geometries allow controlled impedance, 50 ohm lines to be defined if required. Alternative materials include copper as the conductor material and a range of alternative polymers, including BCB (Benzo cyclo butene) and PPQ (Polyphenyl Quinoxaline). This interconnection geometry allows very high interconnection density to be achieved, while the low parasitics of the interconnection traces and the well defined trace impedances provide high bandwidth. The power and ground plane structures provide high performance power and ground connections locally at each mounted device.
A very important and unique capability of the MCM-D substrate option lies in the ability to integrate the full range of thin film resistor, capacitor and inductor components into the MCM-D substrate structure as required, for example for line termination, decoupling, biassing, matching, filtering and related functions.